I used to think I understood global warming. CO2 is a "greenhouse gas" because it is transparent to visible but absorbs infrared. Light passes through the atmosphere and hits the ground, where it re-radiates as infrared. CO2 in the atmosphere keeps it from radiating back out into space, and--voila!--the Earth gets warmer.
That's true as far as it goes--but does that mean that more CO2 means more warmth? The graph above shows why that is a very important question. Adding more CO2 to the atmosphere is like putting more paint on a windowpane--once you've absorbed the infrared radiation coming up from the ground, you can't absorb much more of it.
The claims that increasing CO2 will lead to increasing temperatures are based on hypothetical "feedback" effects, such as "warmer air evaporates more water, which is also a greenhouse gas." But that argument would seem to work for any kind of warming--solar flares, volcanic eruptions, and so forth.